Coating



March 1, 1955 R J. CLQUGH ET AL 2,703,334

COATING Original Filed June 30, 1950 2 Sheets-Sheet 1 FIG.2

ATTORNEY March 1, 1955 P. J. CLOUGH ET AL 2,703,334

COATING Original Filed June 50, 1950 2 Sheets-Sheet 2 )FIG. 5

IN V EN TORS Ph/bp bd/eg 2170' mm M QBNEY COATING Philip J. Clough, Reading, and Philip Godley 2 nd, Lexington, Mass., assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Original application June 30, 1950, Serial No. 171,432. Divided and this application July 14, 1950, Serial No. 173,922

8 Claims. (Cl. 13-22) This invention relates to coating and more particularly tovacuum deposition coating wherein aluminum is evaporated in a vacuum and condensed on a substrate. This invention is particularly directed to sources of aluminum vapors and methods of manufacturing such sources. This application is a division of the copending application of (Zlough et al.,Serial No. 171,432, filed June 30, 1950, now Patent No. 2,665,227, granted January 5, 1954.

[t is aprincipal object of the present invention to provide a new and improved source for heating aluminum in a vacuum to generate aluminum vapors to be used in vacuum deposition coating.

Another object of the present invention is to provide an improved source or the above type which is cheap, easy to manufacture, and which has a long life even though used with aluminum at temperatures in excess of about 13o0 C.

Still another object of the invention is to provide such a source which can be readily heated to operating temperatures by its own internal resistance and which can be brought to its operating temperature in an extremely .short period of time.

Still another object of the invention is to provide improved processes tor manufacturing such sources.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the product possessing the Ieatures, properties and the relation of components, and the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description talren in connection with the accompanying drawings wherein:

Fig. l is a diagrammatic, schematic, partially sectional view of. one embodiment of the invention;

Fig. 1A is a fragmentary, enlarged view of a portion of Fig. 1;

Fig. 2 is a diagramamtic, schematic, partially sectional view taken along the line 2-2 of Fig. 1;

Fig. 3 is a diagrammatic, fragmentary, schematic, partially sectional view of still another embodiment of the invention;

Fig. 4 is a diagrammatic, fragmentary, schematic, partially sectional View taken along line 44 of Fig. 3;

Fig. 5 is a diagrammatic, fragmentary, schematic, partially sectional view taken along line 5-5 of Fig. 3; and

Fig. 5A is a diagrammatic, fragmentary, schematic view similar to Fig. 5 but showing a different condition of use thereof.

In one preferred embodiment of the invention there is provided a vacuum chamber in which the substrate to be coated may be moved past a source of aluminum vapors. The present invention is directed particularly to improvements in such aluminum-vapor sources. In one preferred aluminum-vapor source there is prov1ded an elongated rod which is positioned transversely of the path of the substrate during the coating. This rod is preferably one which has at least a surface stratum wh ch is wettable by molten aluminum, the rod preferably being formed of carbon and having a carbide surface stratum United States Patent 0 which is wettable by molten aluminum. Of the carbides,

2,703,334 Patented Mar. 1, 1955 ice the carbides of the group I'Va and group Va metals as hereinafter enumerated are preferred.

For holding the aluminum in contact with the rod, there is preferably provided a groove along the top of the rod, the apparatus being arranged so as to maintain this groove substantially completely filled with aluminum during the coating operation.

in the utilization of the apparatus described above, the vacuum chamber, after insertion of the substrate and aluminum supply, is evacuated to a low pressure on the order of a micron of mercury and then the rod is heated to a high temperature on the order of 1300 to 1500" C. by passing an electric current therethrou'gh. Aluminum is then fed to the rod (if the rod was not initially filled with aluminum) so that aluminum may be evaporated from the rod. The aluminum carried by the groove in the rod, in a preferred embodiment of the invention, wets the whole outer surface of the rod and thus completely surrounds the rod so that aluminum is evaporated from all surfaces of the rod, thereby giving a line vapor source having a high vapor concentration.

In the various embodiments described above, the distribution of the aluminum along the groove in the rod is primarily achieved by the wetting action of the aluminum over the surface of the rod, however it also may be aided by gravity. In this case the rod may run downwardly from the aluminum supply (either solid or liquid) so that a flowing stream of aluminum is maintained in the groove at all times.

Referring now to Figs. 1, 1A, and 2, wherein like numbers refer to like elements in these and the other figures, there is shown a modification of one preferred embodiment of the invention wherein a grooved rod is employed as a source of the aluminum vapors, and aluminum. in wire form is fed to a cool end of the grooved rod. In these figures, 10 represents a vacuum-tight housing defining therewithin a vacuum coating chamber 12 which is arranged to be evacuated by a vacuum pumping system schematically indicated at 14. Within this chamber the substrate 16 to be coated is guided from a supply 17 thereof past a plurality of water-cooled guiding rolls 18 to a take-up spool 19. During the process of the passage from supply 17 to take-up spool 19, the substrate 16 passes in a series of convolutions near a source 20 of aluminum vapors. The guiding rolls 18 are preferably driven and are so arranged that they do not see the vapor source 20. Thus the guiding rolls are not coated by the vapors from the source and are not exposed to direct heat radiation from the source. The source 20 preferably comprises a carbon rod 22 having a groove 24 in the upper stu'face thereof, this groove being preferably maintained full of molten aluminum 26. One end of the carbon rod 22 is preferably supported by a first rod holder 28 While the other end thereof is supported by a second rod holder 30. These rod holders preferably comprise an inner collar element 32 preferably formed of segmental pieces of carbon supported inside an outer copper clamp 34. The rod holders are preferably arranged so that there is a poor electrical contact with the carbon rod and the various elements of the holders so as to generate some heat in the holders. In the preferred embodiment shown, these holders are cooled by cooling coils 36 which are in contact with the lower part of outer clamps 34, these cooling coils conveniently serving as one of the means to which electric current leads 38 are attached. Leads 38 are connected to a suitable source 40 of electrical energy, this source 40 being indicated as a transformer.

The aluminum 26 is preferably supplied from a wire coil 42 thereof, this wire coil being schematically indicated as being mounted upon a spindle 44 driven by a motor 46. This motor is controlled by a control means 48 which may, for example, be a rheostat. This control means 48 is in turn preferably controlled by a currentmeasuring device 5h which is arranged to measure the amount of current. flowing through the leads 38, thus measuring the amount of current flowing through the rod .22 and the aluminum carried thereby.

In the operation of the device shown in Figs. 1, 1A, and. 2, the roll 17 of the substrate 16 is positioned within the vacuum chamber 12 and the substrate is guided around the various rolls 18 and connected to the take-up spool 19. A spool of aluminum wire 42 is positioned on the spindle 44 and one end of the Wire is guided into groove 24 adjacent that end of the rod 22 which is supported by the rod holder 28. Cooling water (at about 40 F. or lower) is then circulated through rolls 18 by suitable piping (not shown) so as to chill rolls 18. The vacuum chamber 12 is then evacuated to a low pressure, on the order of one micron of mercury, by means of vacuum pumping means 14. When the requisite low pressure is achieved the substrate is advanced, preferably by driving the take-up spool 19 and the various guiding rolls 18. Current supply 40 is then energized so that a current flows through the rod 22. Since the resistance of the rod 22 is relatively high, the current-measuring device 50 will indicate a need for aluminum. This indication is transmitted to the controlling means 48, thus energizing the motor 46 and causing aluminum wire to be fed from the spool 42 along the slot 24. In a preferred method of practicing the invention that end of the rod to which the aluminum wire is fed is maintained at a temperature somewhat above the melting point of aluminum (i. e. about 700-800 C.). However, this temperature is not sufiiciently high to cause immediate melting of the aluminum wire due to the high heat conductivity thereof. This temperature is readily controlled by the amount of heat generated in the rod holder 28, due to its internal resistance, and the amount of cooling achieved by cooling coil 36 associated therewith. As the wire is advanced along groove 24 it finally reaches a point, preferably at about the left side of the holder 28, where the wire melts and the aluminum flows along the groove 24. The aluminum moves along the groove 24 by the wetting action of the aluminum on the wettable surface of the rod. As the aluminum wets the rod 22 a portion of the aluminum climbs out of the groove 24 and wets the circumference of rod 24 as indicated at 26a in Fig. 1A. After enough aluminum wire has been fed to the groove so that the groove is substantially completely filled with aluminum, the current flowing through the rod and the aluminum carried thereby will increase to a point where the current-measuring device 50 indicates that no further feed of aluminum is necessary in which case the controlling means 48 will prevent further feed of aluminum by stopping the motor 46.

In a preferred embodiment of the invention described above, the rod 22 preferably comprises carbon. Preferred forms of these carbon rods have a diameter of about A" and a groove 24 of approximately 4;" wide by A" deep. A preferred method of pretreating the rods so as to form a surface stratum which is readily wet by molten aluminum comprises the steps of preparing a slurry of zirconia (ZrOz), coating the rod with this zirconia slurry, drying the thus coated rod for about one hour at 110 C., and then baking the coated rod at less than one micron of mercury of pressure and at a temperature of about 1450 C. for a period of about one-half hour. After baking, the loose scale on the surface of the rod is then removed, most of the zirconia having formed a surface stratum of zirconium carbide on the rod. With a rod of the above type, which is about 4" long between the supports 28 and 30, there is obtained an aluminum evaporation rate greater than about 1 gram per minute when the rod is operated at a temperature of approximately 1400 C. At such a temperature and rate of evaporation of aluminum, the substrate is preferably moved at a speed of about 100-150 linear feet per minute. The above rod. when carrying aluminum, has a voltage drop of about 2.5 volts per linear inch between the rod supports 28 and 30. With this voltage drop the current flowing through the aluminum-carrying rod is about 180 amperes, while the current flowing through the rod without aluminum is approximately 40 amperes. The rod can be brought to its operating temperature very rapidly, the time necessary being only -15 seconds when the applied voltage is 2.5 volts per linear inch. When the applied voltage is higher, the rod may be brou ht to its operating temperature in less than a second. However, in this case it is desirable to reduce the voltage to about 2.5 volts per linear inch when the operating temperature is obtained.

While the invention has been described particularly in its preferred use for the continuous coating of a flexible substrate, it is equally applicable to batch type of coating operations wherein a number of metallic or plastic objects to be coated are mounted within a vacuum coating chamber and are coated by creating an atmosphere of aluminum vapors within the vacuum chamber. In this embodiment of the invention, shown in Figs. 3, 4, 5, and 5A, there are provided a plurality of carbon rods 88, these rods preferably comprising zirconium or other carbide surface strata prepared in a manner similar to that described previously. Each of these rods is supported by rod holders 82, it being preferred that short lengths of rod of about 4-6 inches be employed. These are connected in series to give any desired length requisite for the dimensions of the coating chamber. Each of these rods is preferably provided with a groove 84 at the top thereof, these grooves being of such size as to support a plurality of aluminum pellets 86. As shown more particularly in Fig. 4, each rod 80 is supported at its ends by rod holders 82, these rod holders each comprising a pair of segmental collars 90, a yoke member 92, and a spring clamp 94 for securing the collars around the rod. In the use of the device of the type shown in Figs. 3, 4, 5, and 5A, the objects to be coated are placed in the vacuum coating chamber along with the rods 80 carrying the aluminum to be evaporated. The chamber is then evacuated to the requisite low pressure and current is passed through the various rods 80 by applying a suitable voltage, on the order of 2.5 volts per linear inch, to the rods. This current, as mentioned previously, heats the rod to a high temperature on the order of 1400 0., thereby melting the aluminum pellets 86 and filling the grooves 84 with molten aluminum. In a preferred use of this embodiment of the invention, a portion of this molten aluminum, due to the wetting action thereof on the zirconium carbide surface, spreads out and covers substantially the whole outer surface of the rod 80 as indicated at 86a in Fig. 5A. However, where directional control of the vapors is desired, the groove 84 may be only partially filled so that evaporation of all the aluminum is achieved before any substantial wetting of the outer surface of the rod occurs.

The arrangement of elements shown in Figs. 3 through 5A thus provides a very cheap source of aluminum vapors which may be used almost indefinitely and which can be very readily loaded and reloaded with aluminum pellets. If desired, the rods 80 may be installed in the coating device with the aluminum in solid form completely or partially filling groove 84, this embodiment of the invention eliminating the necessity of loading the individual grooves with pellets prior to the coating operation. However, since the rods will last for a great number of coating operations, they may be readily refilled as soon as all of the aluminum has been evaporated therefrom. While one preferred type of rod 22 has been described in connection with the various embodiments of the invention, other forms thereof may be employed. For example, the rod 22 may comprise an extruded cemented titanium carbide sintered with cobalt. Additionally, other materials of the type described in the Godley et al. application, Serial No. 158,494, filed April 27, 1950, now Patent No. 2,665,226, granted January 5, 1954, may be used in forming the rods 22.

In one preferred method of forming a wettable carbide surface on the carbon rod 22, a molten bath of aluminum and a group IVa or group Va metal, for example zirconium, is maintained in a vacuum. While in this vacuum the rod, whose surface is to be formed into a carbide, is dipped into this molten bath, thereby converting the surface of the rod to the desired zirconium carbide.

Equally other methods of feeding aluminum to the rods may be employed as illustrated in the above Godley et al. application. Similarly, resistance measurements may be made of the coated web as it travels through the coating chamber, these resistance measurements being used to control the rate of speed of the web or the temperature of the rods and the aluminum thereon.

The expression group IVa and group Va metals is intended to include those metals in groups IVa and Va on the Periodic Chart of the Atoms, Henry D. Hubbard, 1947 edition, W. M. Welch Manufacturing Company. These metals are titanium, zirconium, hafnium, vanadium, columbium, and tantalum.

Since certain changes may be made in the above product and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process of forming an elongated carbon wick element which is relatively inert to aluminum at temperatures on the order of 1300 C., the process comprising the steps of forming an elongated groove in said element along the surface thereof, coating said elongated carbon element with an oxide of a metal taken from the class consisting of titanium, zirconium, hafnium, vanadium, columbium, and tantalum, said oxide being reducible by carbon at temperatures on the order of about 1400 C., baking the coated element at a pressure less than one micron Hg abs. and at a temperature of about 1400 C. to convert only a surface stratum of said element to the corresponding metal carbide while retaining the interior of said wick element as elemental carbon with a relatively higher electrical resistance than the surface of said element.

2. The process of forming a wick element which is relatively inert to aluminum at temperatures on the order of 1300 C. which comprises coating an elongated carbon rod with a slurry of an oxide of a metal taken from the class consisting of titanium, zirconium, hafnium, vanadium, columbium, and tantalum, said oxide being reducible by carbon at temperatures on the order of about 1400 C., drying said slurry-coated rod, baking the slurrycoated rod under reduced pressure at a temperature of about 1400 C. to convert only a surface stratum of said rod to the corresponding metal carbide, said pressure being reduced to less than 1 micron Hg abs. during at least some portion of the baking operation to degas the rod, and removing excess of said oxide from the carbidecoated, gas-free rod.

3. A wick element for evaporating aluminum in a vapor deposition device, said wick element being wettable by molten aluminum, being relatively inert to molten aluminum and comprising an elongated rod Whose surface layer consists essentially of a carbide of a metal taken from the class consisting of titanium, zirconium, hafnium, vanadium, columbium, and tantalum, the interior of said rod consisting of elemental carbon and having a relatively higher electrical resistance than the surface of the rod, said rod being essentially free of all volatiles having a vapor pressure greater than about 1 micron Hg abs. at a temperature above about 1400 C. in order to provide an essentially gas-free rod, the surface of said rod being essentially free of gas-producing oxides of said carbidetorming metals.

4. The wick element of claim 3 comprises zirconium carbide.

5. The wick element of claim 3 comprises titanium carbide.

6. The wick element of claim 3 comprises tantalum carbide.

7. The wick element of claim 3 comprises vanadium carbide.

8. The wick element of claim 3 comprises columbium carbide.

wherein said carbide wherein said carbide wherein said carbide wherein said carbide wherein said carbide References Cited in the file of this patent UNITED STATES PATENTS 1,283,285 Pfanstiehl Oct. 29, 1918 1,312,262 King Aug. 5, 1919 2,013,755 Hediger Sept. 10, 1935 2,205,854 Kroll June 25, 1940 2,282,098 Taylor May 5, 1942 2,315,346 Musgrave Mar. 30, 1943 2,479,541 Osterberg Aug. 16, 1949 2,548,897 Kroll Apr. 17, 1951 2,557,530 Bancroft June 19, 1951 2,665,223 Clough et a1 Jan. 5, 1954 2,665,226 Godley et a1 Jan. 5, 1954 2,665,227 Clough et al Jan. 5, 1954 OTHER REFERENCES Strong, Procedure in Experimental Physics; Prentice Hall, Inc., New York, 1946; pages 171-180.

Periodic Chart of the Atoms, Henry D. Hubbard, published in 1947 by W. M. Welch Manufacturing Co.

Richardson and Scarlett, Brief College Chemistry, published in 1942 by Henry Holt and Co., Inc., New York City. 

3. A WICK ELEMENT FOR EVAPORATING ALUMINUM IN A VAPOR DEPOSITION DEVICE, SAID WICK ELEMENT BEING WETTABLE BY MOLTEN ALUMINUM, BEING RELATIVELY INERT TO MOLTEN ALUMINUM AND COMPRISING AN ELONGATED ROD WHOSE SURFACE LAYER CONSISTS ESSENTIALLY OF A CARBIDE OF A METAL TAKEN FROM THE CLASS CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM, VANDIUM, COLUMBIUM, AND TANTALUM, THE INTERIOR OF SAID ROD CONSISTING OF ELEMENTAL CARBON AND HAVING A RELATIVELY HIGHER ELECTRICAL RESISTANCE THAN THE SURFACE OF THE ROD, SAID ROD BEING ESSENTIALLY FREE OF ALL VOLATILES HAVING A VAPOR PRESSURE GREATER THAN ABOUT 1400* C. IN ORDER TO PROVIDE A TEMPERATURE ABOVE ABOUT 1400* C. IN ORDER TO PROVIDE AN ESSENTIALLY GAS-FREE ROD, THE SURFACE OF SAID ROD BEING ESSENTIALLY FREE OF GAS-PRODUCING OXIDES OF SAID CARBIDEFORMING METALS. 